The present invention relates to a vehicular steering device comprising a unit, which changes a transfer ratio between a steering angle of a steering wheel and a steered angle of a steered wheel, in a steering transmission system which couples a steering member, such as the steering wheel, and the steered wheel by the drive of an electric motor.
Up to now, for the purpose of the progress in drivability and safety of a vehicle, for example, an intelligent front steering control (IFS control) has been proposed, the intelligent front steering control which steers a steered wheel (front wheel) by detecting vehicle state quantities such as a yaw rate and a car body slip angle, and feeding back the vehicle state quantities. For example, what is known as an embodiment of such IFS control is a device of embodying such IFS control in a vehicular steering device comprising a variable transfer ratio mechanism which can change a transfer ratio between a steering angle of a steering wheel and a steered angle (this is also referred to as a rudder angle) of a steered wheel according to a vehicle speed etc.
For example, this device makes it possible to perform prompt steering at the time of low speed driving by relatively accelerating a change of a steered angle of a steered wheel to that of a steering wheel, and raises operation stability at the time of high speed driving by relatively slowing the change of the steered angle of the steered wheel to that of the steering wheel. Moreover, this steering device detects vehicle state quantities such as a yaw rate and a car body slip angle, estimates a state of a steering characteristic (oversteer, understeer, neutral steer) on the basis of this detection result, changes the transfer ratio by switching the IFS control according to the steering characteristics at that time, and improves the drivability and stability of the vehicle. Thus, when judging whether the IFS control is to be switched, it is made to switch the IFS control when the detected yaw rate or slip angle exceeds a control start threshold value.
For example, when a steering characteristic becomes an oversteer tendency, a steering device controls a steering wheel so that the steering characteristic may become counter steering, when the yaw rate or slip angle exceeds the control start threshold value.
By the way, there is a case that the stability of a vehicle deteriorates a little although a driver does not sense a danger when he/she rapidly turns or returns the steering wheel on a high μ road, or on a road having a high coefficient of friction. In such a case, the steering characteristic shows an understeer tendency or an oversteer tendency. Moreover, at that time, the steering characteristic becomes an oversteer tendency when the steering wheel is returned, and for example, a value based on the yaw rate or slip angle which is used as a trigger at the time of switching the IFS control may exceed the control start threshold value. Moreover, similarly, when a vehicle is largely circled at low speed at the time of a U-turn, the steering characteristic becomes an oversteer tendency when the steering wheel is returned, and a value based on the yaw rate or slip angle may exceed the control start threshold value. In this way, there is a problem that operability gets worse in an area where the stability of the vehicle deteriorates a little although a driver does not sense a danger since, conventionally, the IFS control is executed, for example, unnecessary counter steering is performed although the grip of the steered wheel is sufficient.